1. Field of the Invention
The present invention relates to a display device utilizing electrical response characteristics of a material. The present invention relates to, for example, a liquid crystal display device or the like.
2. Description of the Related Art
In an active matrix liquid crystal display device which is a typical electro-optical display device, a circuit including a transistor WTr(n,m), a capacitor (also referred to as a storage capacitor) C(n,m), and a liquid crystal display element LC(n,m) as illustrated in FIG. 2A is provided in each pixel.
FIG. 2B is an equivalent diagram illustrating a state where the circuit holds charges. The capacitor C(n,m) has capacitance C1 and resistance R1, the liquid crystal display element LC(n,m) has capacitance C2 and resistance R2, and the transistor WTr(n,m) has resistance R3. The capacitance C1 of the capacitor C(n,m) is usually several times or more as high as the capacitance C2 of the liquid crystal display element LC(n,m).
Ideally, it is desirable that the resistance R1, R2, or R3 be infinite. In such a case, the display element LC(n,m) can hold charges semi-permanently. In other words, display can be performed semi-permanently. In fact, however, these resistance components have finite values, and leakage current flows through resistors. Accordingly, charges stored in the display element LC(n,m) change with time; thus, regular rewriting (or additional writing) is required. A method for stabilizing the potential of the display element LC(n,m) is disclosed in Patent Document 1.
In general liquid crystal display devices, rewriting of images is performed about 60 times per second (60 Hz driving) or more especially in the case of displaying a moving image. In that case, the rewriting is performed every 16.7 milliseconds (one frame). In such frequent rewriting (or short frame period), variation in luminance or the like of a display element in one frame usually cannot be recognized, and the above-described variation in the charge stored in the display element LC(n,m) is hardly problematic.
However, such frequent rewriting is not generally needed in the case of displaying a still image. A driver needs to be driven to inject charges to a display element every time an image is rewritten, which consumes power. A method in which the frequency of rewriting is reduced as much as possible to reduce power consumption is disclosed in Patent Document 2.
A problem in a conventional active matrix liquid crystal display device including a silicon-based transistor (an amorphous silicon TFT or a polysilicon TFT) was the resistance R3 in the equivalent circuit illustrated in FIG. 2B. The resistance R3 which is resistance of the transistor in an off state (i.e., off-state resistance) was lower than the resistance R1 and the resistance R2 by several orders or more of magnitude.
Thus, charges in a liquid crystal display element could not be held for a long time, and the rewriting frequency could only be reduced to once per several seconds at most for the following reason: if rewriting is not performed for a long time, display is greatly deteriorated.
In recent years, research on a transistor using an oxide semiconductor has been advanced. In such a situation, it was found that off-state current in the transistor using an oxide semiconductor can be reduced to be lower than that in a silicon-based transistor by several orders or more of magnitude. Accordingly, the rewriting frequency can be further reduced; thus, a still-image display method in which rewriting is performed at extremely low frequency, for example, once per 100 seconds is considered possible.    [Patent Document 1] U.S. Pat. No. 7,362,304    [Patent Document 2] U.S. Pat. No. 7,321,353